NCP1397A データシートの表示（PDF） - ON Semiconductor

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NCP1397A

High Performance Resonant Mode Controller with Integrated High-Voltage Drivers

ON Semiconductor 

NCP1397A, NCP1397B

APPLICATION INFORMATION

The NCP1397A/B includes all necessary features to help

building a rugged and safe switch−mode power supply

featuring an extremely low standby power. The below

bullets detail the benefits brought by implementing the

NCP1397A/B controller:

• Wide frequency range: A high−speed Voltage Control

Oscillator allows an output frequency excursion from

50 kHz up to 500 kHz on Mlower and Mupper outputs.

• Adjustable dead−time: Due to a single resistor wired

to ground, the user has the ability to include some

dead−time, helping to fight cross−conduction between

the upper and the lower transistor.

• Adjustable soft−start: Every time the controller starts

to operate (power on), the switching frequency is

pushed to the programmed starting value by external

components (RFmin//RFstart) and slowly moves down

toward the minimum frequency, until the feedback loop

closes. The soft−start discharge input (SS(dis))

discharges the Soft−Start capacitor before any IC restart

excluding the restart after Disable is released AND FB

voltage is higher than 0.3 V. The Soft−Start discharge

switch also activates in case the Fault input detects the

overload conditions.

• Adjustable minimum and maximum frequency

excursion: In resonant applications, it is important to

stay away from the resonating peak to keep operating

the converter in the right region. Thanks to a single

external resistor, the designer can program its lowest

frequency point, obtained in lack of feedback voltage

(during the startup sequence or in short−circuit

conditions). Internally trimmed capacitors offer a $3%

precision on the selection of the minimum switching

frequency. The adjustable upper stop being less precise

to $12%.

• Low startup current: When directly powered from the

high−voltage DC rail, the device only requires 300 mA

to startup.

• Brown−Out detection: To avoid operation from a low

input voltage, it is interesting to prevent the controller

from switching if the high−voltage rail is not within the

right boundaries. Also, when teamed with a PFC

front−end circuitry, the brown−out detection can ensure

a clean startup sequence with soft−start, ensuring that

the PFC is stabilized before energizing the resonant

tank. The BO input features a 28 mA hysteresis current

for the lowest consumption.

• Adjustable fault timer duration: When a fault is

detected on the Fault input or when the FB path is

broken, timer pin starts to charge an external capacitor.

If the fault is removed, the timer opens the charging

path and nothing happens. When the timer reaches its

selected duration (via a capacitor on Pin 3), all pulses

are stopped. The controller now waits for the discharge

via an external resistor on Pin 3 to issue a new clean

startup sequence via soft−start.

• Cumulative fault events: In the NCP1397A/B, the

timer capacitor is not reset when the fault disappears. It

actually integrates the information and cumulates the

occurrences. A resistor placed in parallel with the

capacitor will offer a simple way to adjust the discharge

rate and thus the auto−recovery retry rate.

• Overcurrent detection using Fault input: The fault

input is specifically designed to protect LLC

application in case of short circuit or overload. In case

the voltage on this input grows above first threshold the

Itimer current source is activated and Fault timer

capacitor starts charging. Simultaneously the Soft−Start

discharge switch is activated to increase operating

frequency of the converter. The IC stops operation in

case the Fault timer elapses. The Fault input includes

also second fault comparator that:

− Speeds up the fault timer capacitor charging by

increasing the Itimer1 current to Itimer2 – NCP1397A

− Latches off the device – NCP1397B

The second fault comparator thus helps to protect the power

stage in case of hard short circuit (like shorted transformer

winding etc.)

• Skip cycle possibility: The absence of the soft−start on

the Skip/Disable input (in case the VFB > 0.3 V) offers

an easy way to implement skip cycle when power

saving features are necessary. A simple resistive divider

from the feedback pin to the Skip/Disable input, and

skip can be implemented.

• Broken feedback loop detection: Upon startup or any

time during operation, if the FB signal is missing, the

timer starts to charge timer capacitor. If the loop is

really broken, the FB level does not grow−up before the

timer ends charging. The controller then stops all pulses

and waits until the timer pin voltage collapses to 1 V

typically before a new attempt to restart, via the

soft−start. If the optocoupler is permanently broken, a

hiccup takes place.

• Common collector or common emitter optocoupler

connection options: This IC allows the designer to

select from two possible optocoupler configurations.

Voltage−Controlled Oscillator

The VCO section features a high−speed circuitry allowing

operation from 100 kHz up to 1 MHz. However, as a division

by two internally creates the two Q and /Q outputs, the final

effective signal on output Mlower and Mupper switches

between 50 kHz and 500 kHz. The VCO is configured in

such a way that if the feedback pin voltage goes up, the

switching frequency also goes up. Figure 25 shows the

architecture of the VCO oscillator.

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